Liquid crystal displays (LCDs) have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), and video cameras, because of its portability, low power consumption, and low radiation. LCDs are poised to completely replace cathode ray tube monitors and televisions. A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, and a backlight control circuit for controlling the backlight.
Referring to FIG. 4, a typical backlight control circuit 10 includes a pulse width modulation (PWM) integrated circuit (IC) 11, a power source 12, and a control terminal 13. The PWM IC 11 includes an enable terminal 112 connected to the control terminal 13 and a power input terminal 111 connected to the power source 12. The power source 12 provides an operational voltage, for example, 5 volts, to the PWM IC 11. The control terminal 13 receives a control signal from a scalar IC (not shown) of an LCD.
The backlight control circuit 10 is configured to turn a backlight (not shown) of the LCD on or off. If the LCD is turned off, the scalar IC generates a control signal having a low voltage and sends the control signal to the enable terminal 112 via the control terminal 13. The PWM IC 11 is not operational and no pulse signal is outputted. If the LCD is turned on, the scalar IC generates the control signal having a high voltage, for example, 3.3 volts, and sends the control signal to the enable terminal 112 via the control terminal 13. The PWM IC 11 is operational and output pulse signals are outputted to light up the backlight of the LCD.
In some abnormal conditions, such as load shorting or overload, the operational voltage of the power source 12 can be decreased, for example, lower than 5 volts. If a high operational voltage is still provided to the PWM IC 11, the pulse signals outputted from the PWM IC 11 is unsteady, resulting in an unsteady light emission of the backlight. Thus, a reliability of the backlight control circuit 10 is low.
It is desired to provide a new backlight control circuit, which can overcome the above-described deficiency.